Captive airfoil apparatus

ABSTRACT

Disclosed are apparatus and methods for flying a captive airfoil. The apparatus includes an unpowered flyable airfoil craft having laterally extending wings with a tether line attached to each wing near the outer extremity thereof. The tether lines are secured to ground anchor points spaced apart a sufficient distance to provide an included angle therebetween at the aircraft of about 60* to about 90* when the lines are about 20 feet long.

United States Patent [191 Wheat et al.

[ Oct. 14, 1975 CAPTIVE AIRFOIL APPARATUS Inventors: Robert B. Wheat, 1768 Crestridge;

Roy D. Wheat, 1202 Oriole Lane,

both of Garland, Tex. 75042 Filed: July 18, 1974 Appl. No.: 489,502

Related US. Application Data Continuation-impart of Ser. No. 383,330, July 1973, abandoned.

US. Cl 244/154; 244/155 A Int. Cl. A63H 27/08 Field of Search 244/153 R, 154, 155 R,

References Cited UNITED STATES PATENTS 1/1950 Carnwath 244/153 A 3,696,822 10/1972 Mace ..46/77 Primary Examiner-Duane A. Reger Attorney, Agent, or Firm-Jack A. Kanz [57] ABSTRACT Disclosed are apparatus and methods for flying a captive airfoil. The apparatus includes an unpowered flyable airfoil craft having laterally extending wings with a tether line attached to each wing near the outer extremity thereof. The tether lines are secured to ground anchor points spaced apart a sufficient distance to provide an included angle therebetween at the aircraft of about 60 to about 90 when the lines are about 20 feet long.

5 Claims, 3 Drawing Figures U.S. Patent Oct. 14, 1975 3,912,204

CAPTIVE AIRFOIL APPARATUS This invention relates to tethered or captive unpowered aircraft or gliders. More particularly it relates to methods and apparatus for controlling and maintaining an unpowered airfoil in captive flight.

Since ancient times kites of various design have been flown for amusement and other purposes. All kites essentially comprise a relatively lightweight body presenting at least one relatively flat surface upwind at a relatively high angle of attack, an anchor harness such as a string, and some drag-inducing dampening means such as a tail. Kites do not usually employ an airfoil in the conventional sense, but instead employ a relatively flat plate with a high angle of attack presented to relative wind to generate lift.

Since the invention of the airplane and glider using airfoil wings to generate lift, attempts have been made to fly unpowered airfoils in tethered arrangements such as conventionally used for kites. An unpowered craft using an airfoil which can be flown in a tethered arrangement has particular appeal as a novelty or toy if the unpowered craft can be made to simulate'or duplicate the physical appearance of a conventional airplane in flight. However, because of the aerodynamically clean design of airplanes a scale airplane could not heretofore be flown as a captive glider using merely a tether without artificial drag-inducing means such as a tail or the like. Obviously the attachment of a draginducing tail seriously detracts from the desired illusion. However, without artificial drag-inducing means or other stabilizing means scale models of airplanes suffer severe lateral instability and cannot be successfully flown in tethered arrangements.

In accordance with the present invention apparatus is provided for tethering an unpowered airfoil (which may be in the configuration of a scale model airplane) so that the captive craft will take off, climb, cruise and perform acrobatic maneuvers without visible artifically induced drag. In the tethering arrangement of the invention two tether lines are employed; each attached to the aircraft at laterally displaced points near the outer periphery of the airfoil and extending from the airfoil to the ground at a diverging angle of about 60 to about 90. The tether lines are attached near the center of lift of the aircraft and anchored on the ground, the horizontal distance between the ground anchor points being substantially greater than the wingspan of the airfoil. The tether lines, therefore, are in the general form of an inverted V. When tethered in this arrangement a craft in the configuration of a scale model airplane will fly as though it were powered when presented with sufficient relative wind to generate the required lift. The airplane will take off, climb, and level off in a cruise attitude almost directly above the fixed ground anchor points. Lateral spacing of the tie points on the aircraft provides lateral stability and the aircraft flies in a relatively stable condition. By manipulation of the tether lines the aircraft may be made to climb, dive, perform loops and other acrobatic maneuvers.

Other features and advantages of the invention will become more readily understood when taken in connection with the appended claims and attached drawings in which:

FIG. 1 is a side view of a typical airfoil illustrating the attachment of the tie points in accordance with the invention;

FIG. 2 is a plan view of the underside of a typical model airplane showing the position of the tie points; and

FIG. 3 is a diagrammatic illustration of the harness arrangement of the invention.

Many attempts have been made to provide unpowered but flyable captive scale model airplanes. However, because of the clean aerodynamic design of aircraft, unpowered models could not heretofore be flown in a tethered arrangement without providing some artificially induced drag means, such as a tail, which seriously detracts from the desired illusion. Without artificially induced drag, the aerodynamically clean unpowered airfoil is highly unstable.

In accordance with the present invention lateral stability of the unpowered airfoil is provided through the use of a pair of tether lines, one attached to the airfoil near the outer extremity of each wing tip. Further stability is provided by balancing the aircraft to have an aft center of gravity.

The arrow in FIG. 1 illustrates the lift component of an airfoil in level flight. The lift component is distributed over the top surface of the airfoil. Center of lift, of course, varies with the angle of attack. In accordance with the invention, however, the tie point is attached to the underside of the airfoil 10 directly below the center of lift when the airplane is in level flight. It will be understood that an aircraft may have more than one lifting surface. The center of lift, for purposes of this application, is defined as the fore-to-aft point on the aircraft which has equal lift on either side thereof when the aircraft is in level flight.

As illustrated in FIG. 2 the tie points 11, which may be simple eyebolts or the like, are attached to the underside of a model airplane wing at points near the outer extremities of the wing. The scale model airplane 12 may be of any conventional airplane, helicopter, autogyro, ornithopter or sailplane design but is preferably of a high lift, high drag design such as vintage aircraft of the l9l0l930 era.

The scale model, of course, is a completely flyable craft but without an engine. Therefore, it must be properly weighted to provide the appropriate balance. In the preferred embodiment the empennage is also an airfoil providing lift and the craft if weighted so that the center of gravity of the craft is aft of the center of lift when the craft is in level flight. In this configuration, the empennage provides an automatic dampening. For example, if relative wind decreases below that required to maintain cruise condition while the craft is in level flight, the aft center of gravity will cause the craft to assume a nose-up attitude. The angle of attack is thereby increased. Furthermore, the center of lift moves forward and the aircraft attempts to climb. However, if insufficient relative wind is provided to cause the craft to climb, the craft begins to descend with a high angle of attack until it lands.

In powered aircraft the center of gravity is preferably directly below the center of lift when the craft is in level flight. Ordinarily the center of gravity is about onethird of the distance aft of the leading edge of the fore to-aft chord of the wing. In the preferred embodiment of the invention, however, the center of gravity (in level flight) is displaced aft to approximately 50% to 60% aft of the leading edge of the'fore-to-aft chord of the wing. Weighting the craft to have a slightly aft center of gravity may be easily accomplished without altering the external appearance of the craft. Therefore, authentic scale models of powered and unpowered craft using airfoils for lift may be tethered in accordance with the teachings of this invention and flown without onboard powerplants.

For authenticity'of appearance, a free spinning prop may be attached which acts as a windmill providing the illusion'of a powered airplane. The propeller also give visible and audibleindication of the wind velocity.

Tether lines 20 and 30 are attached to the tie points 11 and, as illustrated in FIG. 3, the opposite ends are attached to suitable anchor points A and B.

:For launching the airplane is placed at the desired take-off position facing directly into'the wind as illustrated by W in FIG. 3. The lines 20 and 30 are drawn taut and anchored at positions A and B in front of the aircraft. For take-off the lines A and B deviate from center at an angle of about 30 to about 45 as indicated by X in FIG. 3. The lines 20 and 30 may be of any desired length, but for take-off should be approximately 20 to 30 feet. The lines 20 and 30 are maintained taut and, when the aircraft is subjected to sufficient wind to generate the required lift, the aircraft rotates and climbs in the manner of a normal aircraft. Since positions A and B are fixed, the aircraft will climb and move, forward to a position slightly downwind from apoint'vertically above the midpoint between positions A. and B.v As the aircraft reaches the top of the climb it levels off in the normal manner to a cruise attitude.

It will bereadily understood that lines 20 and 30 need not be permanently affixed to anchor points A. and B. Either or both of the'anchor points may be hand-held. Likewise, one or both may be a pulley arrangement whereby both lines may beoperated by a single operator; The operator may be positioned between anchor points A and B to pay out any desired length of line to the aircraft. As the aircraft is allowed additional line it rotates to a climb attitude and again climbs to the height allowed by the length of the lines 20 and 30.

The lengths of lines 20 and 30 are not critical. By reeling out line 30 anchored at B, the operator may hand launch the aircraft from point C, paying'outline 20 and moving to point A. This, of course, also holds true for the opposite (reeling out line 20, hand launching aircraft at C, and moving to point B while paying out line 30). By restraining the aircraft at point C on the ground with chocks or hooks, line 20 laid out to A, and the operator at point B with line 30 in hand, the aircraft may be launched with a tug on line 30.

A hand-held hand-cranked reel is very advantageous for the operation of the aircraft, especially in performing aerobatic maneuvers. A finger or squeeze type brake on the reel drum enhances its'operation. If desired, anchor points A and B may both be pulleys and the operator positioned between points A and B'. Lines 20 and 30 may then be either hand-held or jointly operated by means of a dual reel.

It should be noted that as the airplane climbs higher, the anchor points A and B need not be moved further apart. At higher altitudes the line trailing from the tie points to the anchor points provide sufficient drag to stabilize the airplane and, if the anchor points are spaced a sufficient distance apart for take-off, the width generally need not be'increased as the airplane climbs to a higher altitude. Accordingly, as the airplane climbs to heights to over a hundred feet or more, lateral stability is provided by the drag of tether lines 20 and 30 trailing downwardly from the aircraft wings. However, since the line is quite thin it is practically invisible so the airplane continues to operate giving the illusion of being completely free.

It should be observed that by manipulation of the tether lines 20 and 30 the aircraft can be made to perform acrobatic maneuvers. For example, by releasing pressure on the lines 20 and 30 the aircraft will rotate to a climb attitude and begin to climb. By appropriately relaxing and increasing pressure on the tether lines the aircraft can be made to climb or dive and therefore perform loops. The maneuvers performed, of course, will be determined by the design of the airfoil, the relative wind, and the skill of the operator.

While the invention has been defined with particular reference to scale model aircraft, it will be readily apparent to those skilled in the art that other airfoils may likewise be flown using the method described. Likewise, whereas the ground anchor points have been described as fixed, it will be apparent that the anchors may be assistants or other operators holding lines or pulleys byhand, or simply weighted blocks or the like which may be readily moved by the operator but not by the force exerted thereon by the lift of the airfoil.

It is to be understood that although the invention has been described with particular reference to specific embodiments thereof, the forms of the invention shown and described in detail are to be taken as preferred embodiments of same, and that various changes and modifications may be resorted to without departing from the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

' 1. Tethered flying apparatus comprising:

a. an aircraft having substantially laterally extending airfoil wings capable of producing lift and a center of gravity aft of the center of lift when said aircraft is in level flight, and

b. two tether lines, the first end of each tether line attached to one of said wings at a point near the outer end thereof and the second end of each tether line held at a fixed point so that the angle formed by the intersection of each of said tether lines and the centerline of said aircraft is about 30 to about when each of said tether lines is about 20 feet in length.

2. Apparatus as defined in claim 1 wherein said points near the outer ends of said wings are substantially directly below the center of lift when said aircraft is in level flight.

3. Apparatus as defined in claim 1 wherein the center of gravity of said apparatus is approximately to aft of the leading edge of the fore-to-aft chord of the 'wing.

4. The method of controlling an unpowered tethered airfoil having laterally extending wings comprising:

a. weighing said airfoil so that the center of gravity thereof is aft of the center of lift when said airfoil is in level flight,

b. attaching one end of first and second tether lines near the outer'ends of each wing, respectively, substantially directly below the center of lift when said airfoil is in level flight,

c. positioning the fore and aft centerline of said airfoil parallel to the prevailing wind,

6 d. extending each of said tether lines upwind from 5. The method as set forth in claim 4 wherein said opalrfoll at an angle from the centerline of Said posite end of said first tether is secured to a first anchor airfoil at about 30 to about 45, and

e. securing the opposite ends of said first and second tether lines at anchor points approximately 20 feet 5 extended as the airfoil Climbsfrom said airfoil.

point, the airfoil launched, and the second tether line 

1. Tethered flying apparatus comprising: a. an aircraft having substantially laterally extending airfoil wings capable of producing lift and a center of gravity aft of the center of lift when said aircraft is in level flight, and b. two tether lines, the first end of each tether line attached to one of said wings at a point near the outer end thereof and the second end of each tether line held at a fixed point so that the angle formed by the intersection of each of said tether lines and the centerline of said aircraft is about 30* to about 45* when each of said tether lines is about 20 feet in length.
 2. Apparatus as defined in claim 1 wherein said points near the outer ends of said wings are substantially directly below the center of lift when said aircraft is in level flight.
 3. Apparatus as defined in claim 1 wherein the center of gravity of said apparatus is approximately 50% to 60% aft of the leading edge of the fore-to-aft chord of the wing.
 4. The method of controlling an unpowered tethered airfoil having laterally extending wings comprising: a. weighing said airfoil so that the center of gravity thereof is aft of the center of lift when said airfoil is in level flight, b. attaching one end of first and second tether lines near the outer ends of each wing, respectively, substantially directly below the center of lift when said airfoil is in level flight, c. positioning the fore and aft centerline of said airfoil parallel to the prevailing wind, d. extending each of said tether lines upwind from said airfoil at an angle from the centerline of said airfoil at about 30* to about 45*, and e. securing the opposite ends of said first and second tether lines at anchor points approximately 20 feet from said airfoil.
 5. The method as set forth in claim 4 wherein said opposite end of said first tether is secured to a first anchor point, the airfoil launched, and the second tether line extended as the airfoil climbs. 